1. Field of the Invention
The present invention relates to class D amplifiers, especially to a circuit for modulating the circulating current to reduce the distortion of a Class D amplifier.
2. Description of the Prior Art
With Class D amplifiers the “carrier” frequency (period of the PWM ramp generator) is typically in the range of 200 kHz to 2 MHz. At full output power the signal is high all the time; at 50% power, the signal is high 50% of the time and decreases linearly with the desired audio level. Since the output power is proportional to the output pulse width, it follows that signal distortion is proportional to the error in the effective pulse width compared to the desired pulse width. At large signal levels, this error is small; however, for small input signals the error is large. For typical CMOS processes that are utilized for Class D amplifiers, the smallest pulse that can be output is about 50±10 nsec. Since the human ear has a very wide dynamic range, to hear undistorted output signals requires a total error to signal ratio (called THD+N total harmonic distortion plus noise) of about −60 dB. To achieve this with a PWM system, the linear ratio of minimum pulse width to maximum pulse width needs to be 1000:1, e.g. for a 2 usec period the smallest pulse needs to be 2 nsec. Obviously, this is not possible in a process that has a 50 nsec minimum pulse width. For typical Class D amplifiers the uncorrected large signal distortion will be around −35 dB to −45 dB, but the percentage distortion climbs rapidly with decreasing signal level. There are a number of techniques which seek to linearize the output stage so that distortion is reduced and small signal performance is improved. The three main error correction techniques are:
(a) Feedback from the Class D output to an input integrator;
(b) Dummy correction pulses (i.e. for a 1 nsec pulse we output positive 51 nsec pulse and a negative 50 nsec pulse);
(c) Sigma-Delta noise shaping plus feedback.
To “noise shape” the output so that wider pulses are used, the signal error components are shifted by the noise shaping function to be above the audible frequency band. Noise shaping the signal typically improves the linear range by about 20 dB over that which is achieved with simple PWM. This results in large signal distortion components at −50 dB to −60 dB.
For state 3 and state 4 PWM systems disclosed in U.S. Pat. No. 6,211,728, the ON pulse width is proportional to signal strength and during the OFF period the current induced in the filter inductors by the ON pulse is circulated either through the NMOS or PMOS devices in the H-bridge. These are known respectively as LOW side circulation and HIGH side circulation.